1. Field of the Invention
The present invention relates to a wireless communication system, a wireless base station, a wireless terminal and a method of wireless communication which perform wireless communication between the wireless base station and a plurality of wireless terminals.
2. Related Art
There are the following three frame formats in a wireless communication system composed of a wireless base station and wireless terminals.
1) A unicast format in which the wireless base station discriminates the wireless terminals, and transmits individual wireless frames to the wireless terminals.
2) A multicast format in which although the individual wireless terminals are discriminated, the wireless frames are collectively transmitted to all the wireless terminals.
3) A broadcast format in which the wireless frames are transmitted without discriminating the wireless terminals
In the unicast format, it is possible to perform transmission suitable for the wireless terminals depending on propagation paths for the wireless terminals, and to assure received qualities of the wireless terminals. However, since a traffic increases in proportion to the number of the wireless terminals, there is a drawback in which frequency availability deteriorates.
On the other hand, in the multicast format and the broadcast format, since the wireless frames are transferred in block to a plurality of wireless terminals, it is possible to improve the frequency availability. However, in order to assure the received qualities of the wireless terminals, the wireless frames have to be transmitted in accordance with the wireless terminal having poor received properties, thereby deteriorating power efficiency. Furthermore, it is difficult to apply retransmitting control techniques for transmitting the wireless frames to all the wireless terminals, and the wireless frames has to be repeatedly retransmitted so that all the wireless terminals can correctly receive the wireless frames. Because of this, control efficiency of the wireless base station deteriorates due to retransmission.
Japanese patent Laid-Open No. 2002-132613 discloses techniques in which a representative user terminal is selected from a plurality of user terminals, and the representative user terminal communicates with a distribution server. More specifically, the distribution server divides download data according to information from the representative user terminal, transmits the divided data to the user terminals, and combines the divided data received by the user terminals.
In the techniques disclosed in this document, a plurality of user terminals cooperatively download data. Because of this, if some user terminals cannot correctly download data, it becomes impossible to combine data and to download reliable data.